The present invention relates to data transmission equipment, and especially to such equipment which is flexible and economical. The invention also concerns signalling techniques for use in association with such equipment.
The main trend in telecommunications at present is towards digital systems, and especially to such systems in which speech is conveyed in time division multiplex (TDM) manner using pulse code modulation (PCM). For speech handling in Great Britain the speech is usually sampled at 8 KHz, and each speech sample is conveyed by eight binary bits so that we get a 64 Kbit/sec channel. For an inter-exchange trunk it is usual to convey 32 such channels in TDM manner, which gives a 2048 Kbit/sec trunk, often referred to as a 2 Mbit/sec trunk. The numbers of channels may vary; thus in USA most PCM systems are 24 channel systems. The present invention is applicable to such systems with different numbers of channels.
It will be appreciated that other bit rates exist on subscribers' channels especially where data is handled. However, it is still common practice to assemble the channels onto the 2048 Kbit/sec trunks. Thus when all the bandwidth is in use, such a trunk may be handling less than the full number of 31 subscriber channels. Note that one of the 32 channels on the trunk is usually used for synchronisation, and another channel may be used for common channel signalling. Assembling the lower bit rate channels onto the higher bit rate trunks involves multiplexing; in addition it is necessary to be able to "disassemble", i.e. to demultiplex, from the higher bit rate trunks to the lower bit rate channels.